Dark Energy

So I've already talked a bit about Dark Matter... but something even more crazy and mysterious is Dark Energy. It accounts for an enormous 74% of our universe! When combined with Dark Matter, this means that around 86% of our universe has not been seen/discovered yet! According to NASA, "more is unknown [about dark energy] then is known." We know that our universe is continually expanding, and has been since it was formed. What is, perhaps, even more surprising is that this expansion has actually grown faster over time! Previously it had been thought that the expansion rate would slow down, and eventually stop due to gravity. So what is causing this expedited expansion? Scientists call this Dark Energy. When the universe first came into existance, it was affected primarily by dark matter. In present times, however, dark energy has a much greater effect. 
If you look at this "cup-shaped" figure, the bottom represents the formation of the universe, which is expanding, but at a much slower rate than the top of the cup. With the switch over to dark energy (occuring slightly more than half-way up), expansion sped up dramatically. Today, we are at the brim of the glass, and possibilities for expansion are endless! Many scientists actually believe that once day, as the universe as ripped apart even more, our Milky Way Galaxy will be its own little island, with all the other Galaxies and other celestial bodies we can see today being pulled even further away.
Dark energy, of course, is only one of 3 theories about the expansion of the universe, but it is the most weidely believed theory in today's scientific society.